This invention concerns light grids of the type which have a housing in which radiation emitters and transducers are arranged spaced apart from each other in an extruded profile adapted to receive and hold a longitudinally movable electronic component board.
Light grids are in wide use for recognizing and measuring objects on conveyors, for baggage control on baggage conveyors, for controlling the doors of elevators, for protecting hands and bodies in the vicinity of machinery, to control access for persons and vehicles, and so forth. Such light grids have frames on which associated emitting units and receiving units are mounted. The emitting units send light rays, typically infrared light, which strike the associated receiver units. Interruptions in the radiation are detected and used.
Each emitting unit has an optoelectronic element which is spaced apart in the direction of the light from an optical functional element. At the emitting unit, an optoelectronic transducer emits light which is focused on a corresponding receiver unit by the optical function element, such as a lens and/or an adjustable aperture. The receiver unit also has an optical function element which concentrates the light on an optoelectronic transducer.
The optoelectronic elements and the associated optical function elements of the individual emitter and receiver units must be mounted in a housing. They must be adjustable and be highly accurately positioned relative to each other. In this regard, it is known to mount the required optoelectronic elements and the associated optical function elements with, for example, a modular tubular structure in which the optoelectronic elements and the associated optical function elements are adjustably positioned. These modules are then installed in a housing, where they must be accurately positioned relative to each other and relative to the housing. The necessary adjustments of the optoelectronic elements and the optical function elements that must be made in such a module and on the housing are time-consuming and costly.
It is also known to form the housing as a U-shaped profile where the modules can be inserted through the open front end and then fastened, for example with a snap connection. Such U-shaped housings, however, have low torsional stiffness. Increasing the torsional rigidity of such an extruded profile requires heavier material cross-sections.
It is also known to employ a closed profile as the housing for a light grid. In such a case, the modules must be inserted in the longitudinal direction of the profile. Especially for great length, this is time-consuming and makes an accurate positioning of the modules in the housing difficult. In addition, this requires the machining of the extruded profile, which increases manufacturing costs.
It is also known to use tub-shaped plastic housings where modules can be readily installed. However, such housings require additional metal strengthening and rigidifying to attain the required stability and positional accuracy.